In recent years, with the rapid development of wireless communication throughout the world, radio frequency spectrum resources are becoming increasingly inadequate. In order to make more effective use of the limited frequency spectrum resources, many wireless communication systems employ methods of linear modulation, by which the utilization ratio of frequency spectrum is higher. Because both the phase and the amplitude of a linear modulated signal carry useful information, any nonlinear amplification for this signal may cause an increase in bit error rate and an interference for adjacent radio frequency channel, and therefore it is necessary to ensure the linearity of amplification for the linear modulated signal.
The predistortion method is a common linearized method. Compared with traditional feedforward techniques, it can have higher power efficiency and does not need sophisticated manual modulation at the same time thereby suiting a large-scale production. With the rapid development of digital signal processing techniques, digital predistortion techniques are also improved gradually. The digital predistortion techniques can be mainly divided into two kinds: one is adding predistortion compensation signal on a digital baseband, the other is controlling vector (phase, amplitude) modulation devices by using digital baseband signal to generate appropriate distortion compensation component.
Substantially, there is no essential distinction between the technique for compensation (predistortion) before distortion of an amplifier and the technique for compensation (feedforward) after distortion of the amplifier, and their results are supposed to be similar. However, memory effects exist in the amplifier, therefore the linearized result of predistortion is greatly reduced. In time domain, when there are memory effects, the distortion characteristic of the amplifier has relations with not only current input values but also previous input values; in frequency domain, memory effects imply that the amplitude of nonlinear distortion component of the amplifier will vary with modulation frequency of input signals. The existence of memory effects affects greatly the result of distortion prediction of the predistortion method, but this is not a problem for the feedforward technique in which the compensation is made after the amplifier is distorted. Therefore actually, the linearized bandwidth and linearized performance achieved by the feedforward technique are generally better than that achieved by the predistortion technique.
Another problem facing the predistortion technique is how to achieve satisfying predistortion performance within a limited space and limited resources (e.g., power, computing capacity, etc.) of mobile communication devices (e.g., mobile terminals). It is difficult for current analog predistortion to achieve the performance and stability required by applications, and common digital predistortion methods may over-consume resources.
The envelope injection technique is a linearized method with a simple implementation and reliable performance, and it uses a low frequency signal to perform distortion compensation functions. Specifically, according to this technique, the low frequency signal that is directly proportional to power envelope of input signals is injected into the amplifier and frequency mixing is performed for said envelope signal and an input original radio frequency signal using effects of second-order (even order) nonlinear frequency mixing of the amplifier itself, thereby generating a compensation signal which has the same amplitude and the opposite direction of an inter-modulated distortion signal of the amplifier itself. When taking appropriate means, the envelope injection signals may also compensate memory effects of the amplifier. In a desired case, the envelope injection may achieve almost complete distortion compensation, but in actual systems, some non-desired factors limit results of compensation, for example, phase distortion component generated by nonlinearity of input capacitance of the amplifier will increase as input power increases, which will cause the angle between compensation component and distortion component to become bigger and bigger. For this reason, the envelope injection can only achieve partial counteract of inter-modulated distortion. In order to solve the problem, the most important is to achieve “phase adjustment” of envelope injection compensation signal.
K-K M. Cheng and others from the Chinese university of Hong Kong have introduced multipoint envelope injection technique to solve the problem. This technique is simultaneously injecting two envelope signals at different positions in a cascade amplifier to obtain two degrees of freedom of control to compensate distortion signals having independent amplitude and phase. However, the phase difference between the distortion compensation components introduced by adopting this technique is usually small, therefore, it is possible for certain distortion components at specific angle to compensate ineffectively. Changing the positions of injection for envelope signals is only a method solving the problem temporally not fundamentally. Thus, the key to the problem is how to superpose two independent envelope injection signals to generate compensation signals.